If you add liquidity you get paid by exchanges to do this, if you take liquidity you pay for this privilege. Different Routes have different payments or costs.
As an example, if you want to buy and sit on "Bid" waiting for a buyer and get filled you are adding liquidity and will be paid for doing so. Alternatively, if you want to buy and you take the "Ask" you are getting filled straight away but you will be taking liquidity and will pay for this.
ECN rebates are automatically applied at the end of each trading day.